The invention relates to a process for extruding or pressing out a wet or moist material, such as a filter cake, a paste, a feedstuff, etc. from openings and in particular for granulating a paste, in which the material is conveyed through at least one opening using a predeterminable contact pressure. The invention also relates to an apparatus for performing the inventive process, in which the material is conveyed through at least one opening with the aid of a conveying means.
Such a process and a corresponding apparatus are known from the journal "Chemie-Ingenieur-Technik", 49 (1977), No. 5, pp. 374 to 380.
Although the known apparatus functions correctly, it suffers from various disadvantages. Thus, the openings in the perforated plate tend to increasingly become blocked in operation. This tendency occurs independently at different individual openings. If no further paste is extruded over several pressing processes, the paste dries from the outside of the perforated plate. As a result a relatively strong connection is formed between the paste and the perforated plate and in particular with the edge of the holes. If such a paste attachment takes place, there is virtually no chance of self cleaning of said hole.
If a larger number of holes become clogged, the following drying means, e.g. a belt dryer becomes non-uniformly covered. Thus, short-circuit flows of the hot air occur with all the problems known from the prior art and referred to in the aforementioned publication. In most cases the perforated plate must be cleaned with a high pressure water jet during an operational stoppage.
There are various ways in which the openings in the perforated plate can become clogged. The more rigid lumps may come from an upstream filter and have not been correctly mixed by stirring, so that the mass is not sufficiently homogeneous. It is also possible that in the case of coarser particles in the mass or paste a self-supporting dome may form over an opening in the perforated plate. Then, when the contact pressure is applied, the mass will only deliver liquid through the dome, which leads to a further local thickening of the mass causing a more intense self-locking effect.
As fundamentally any mass or material has a certain initial shear stress, it is not possible to move a press die or displacement body up to the perforated plate with a zero spacing. A relatively narrow zone filled with material is always left between the perforated plate and a press die or a displacement body. In other words between the pres die or displacement body and the perforated plate there is always material which cannot flow.
In a very large number of cases a so-called paste deformer, i.e. an apparatus for performing the inventive process of the aforementioned type is placed between a filter and a belt dryer, but then the following problem occurs. In order to save thermal energy in the dryer, every effort is naturally made to mechanically dry the material or paste of the filter to the greatest possible extent. However, there is a definite limit to the carrying out of corresponding measures, because the material or paste becomes more rigid with decreasing moisture content. Finally, with increasing toughness, it can arise that it is no longer possible to press the material through the openings in a perforated plate. However, a relatively tough paste could still be extruded through a perforated plate if the openings in the latter were sufficiently large. However, there is a fundamental requirement of operating with openings which are as small as possible. Due to the dispersability of the paste and the drying speed in the following drying process, it is necessary to work with minimum sized openings in the perforated plate.
It would fundamentally also be possible to liquefy the paste after taking from the filter by applying mechanical energy (stirring and/or mixing), but then the initial shear stress would drop to such an extent that the paste would drip in liquid form through the perforated plate and no aeratable heap would form on the belt dryer.